1. Field of the Invention
This invention relates to composite abrasive particles for use in magnetic abrasive polishing which effects deburring and/or surface finishing of machine parts and a process for preparing the same.
2. Prior Arts
Recently, a magnetic abrasive polishing has been developed and has attracted the attention of many researchers (for example, Takeo Shinmura, "Magnetic Abrasive Process," KINO ZAIRYO (Functions & Materials), Jan. 1985, pp.11-23 issued from K.K. CMC; Japan). According to this process, when the work and magnetic abrasive particles responsive to magnetism are dispqsed between magnetic poles, the magnetic abrasive particles are placed under restraint by a magnetic field. If the work is subjected to rotation or vibration when the abrasive particles are in this restraint, an abrasive effect is exerted like a brush. The magnetic abrasive particles used in this method must have both ferromagnetism and high hardness and they are usually a composite of iron and a material of high hardness such as aluminum oxide (alumina), titanium carbide, etc. As a process for preparing such composite particles, there have been known a process in which finely broken particles of iron and the materials of high hardness are sintered in a furnace after pressure molding or sintered while being hot pressed, a process in which a metallic alloy (for example, Al-Si-Fe) is internally nitrided, a process in which a carbide is produced from a melt of metallic alloy and carbon, a process in which a composite of titanium carbide and iron is prepared by utilizing an exothermic reaction between carbon and titanium (A. B. Lyashchenko et al., Poroshkovaya Metallurgiya 9 44-48 (1983)), or the like. In any process as described above, the produced composite material is subjected to breaking into an appropriate particle size to provide magnetic abrasive particles.
Among these processes, the process in which sintering is carried out after pressure molding and the process in which sintering is effected while being hot pressed may be called a sintering process. This sintering process needs an expensive sintering furnace and requires considerable troubles for breaking the composite prepared by the sintering process into a desired particle size. In the case of the composite which has been prepared at a relatively low temperature or within a relatively short period of time, the breaking into the desired particle size is not so difficult, but the bonding between iron and the material of high hardness is rather weak and iron and the material of high hardness are liable to be separated during the breaking process or within a short time after the composite has been started to be used in an abrasive polishing machine. Whereas, when the sintering is effected at a high temperture or for a long time, the grains of iron and the material of high hardness are consolidated and grow. In this case, the bonding between iron and the material of high hardness are strong and growth of the grains is also caused, so that the breaking is difficult and it is almost impossible to obtain finely broken particles.
The process employing the inner nitriding of metals or the process of preparing the abrasive particles from the melt needs expensive equipment and is rather poor in the yield. Thus, these processes are not economical. In addition, the breaking is extremely difficult and it is almost impossible to obtain finely broken particles as small as 44 .mu.m or less.